1. Field of the Invention
This invention generally relates to laser scanning systems for reading indicia having portions of different light reflectivity such as bar code symbols and, more particularly, to a lightweight, multi-component, portable laser diode scanning head supportable by a user and aimable at each symbol to be read and, still more particularly, this invention relates to increasing the visibility of the laser beam emitted by the laser diode in the head to facilitate aiming the laser beam at the symbol.
2. Description of Related Art
Various optical readers and optical scanning systems have been developed heretofore to optically read bar code symbols printed on labels affixed to objects in order to identify the object by optically reading the symbol thereon. The bar code symbol itself is a coded pattern comprised of a series of bars of various widths, and spaced apart from one another to bound spaces of various widths, said bars and spaces having different light-reflecting characteristics. Such readers and systems electro-optically decoded the coded patterns to a multiple alpha-numerical digit representation descriptive of the object. Scanning systems of this general type, and components for use in such systems, have been disclosed, for example, in U.S. Pat. Nos. 4,251,798; 4,360,798; 4,369,361; 4,387,297; 4,593,186; 4,496,831; 4,409,470; 4,460,120; 4,607,156; 4,673,805; 4,736,095; 4,758,717; 4,760,248; 4,816,660; 4,816,661; 4,806,742; 4,835,374; 4,845,350 and 4,871,904, as well as in U.S. patent application Ser. Nos. 7,775 and 193,265; all of which have been assigned to the same assignee as the instant application and are incorporated herein to show the state of the art.
As disclosed in some of the above patents and applications, a particularly advantageous embodiment of such a scanning system resided, inter alia, in optically modifying and directing a laser light beam from a hand-held head which was supported by a user; aiming the head and, in some cases, the laser beam itself at a symbol to be read; repetitively scanning the laser beam and/or the field of view of a detector across the symbol; detecting the laser light reflected off the symbol during scanning; and decoding the detected reflected light.
When the laser beam was generated by a helium-neon gas laser which emitted a red laser light at a wavelength of about 633 nm, the red laser light was readily visible to the user and, thus, the user, without difficulty, could properly aim the head and position and maintain the emitted red laser light on and across the symbol during the scanning.
When the laser beam was generated by a semiconductor laser diode which emitted infrared laser light at a wavelength of about 780 nm, the infrared laser light was, for all practical purposes, invisible. As explained, for example, in U.S. Pat. No. 4,760,248, the user could not see the invisible infrared laser light when the scanning occurred in a well-lit environment and at rapid scanning rates. Aiming of the head relative to the symbol was thus rendered difficult because, simply put, the user could not see the infrared light. The user was required to hunt around by trial and error, hope that the infrared light was eventually properly positioned on and across the symbol, and wait until the system advised him, typically by the lighting of an indicator lamp or by the sounding of an auditory beeper, that the symbol had indeed been successfully decoded and read.
To overcome the inefficiencies and time-consuming nature of the trial-and-error hunting technique, U.S. Pat. No. 4,835,374 described an aiming light arrangement for visually locating and, in some cases, tracking each symbol when the head emitted and/or received such invisible laser light. Although generally satisfactory for its intended purpose, the aiming light arrangement added weight, size, cost and power consumption to the head and, in the context of making the head as low in weight, as small in size, as inexpensive in cost, and as energy efficient as possible, was a less-than-ideal solution.
With the advent of laser diodes which emit laser light at a wavelength of about 670 to about 680 nm, the emitted laser light is at least marginally visible to the human eye. By way of comparison, laser light at a wavelength of 670-680 nm is about one-tenth as visible to the human eye as the red laser light emitted by a helium-neon gas laser at a wavelength of about 633 nm, but is more visible than the infrared laser diode light whose wavelength is about 780 nm. Nevertheless, in use, at rapid scanning rates on the order of 40 scans per second and in well-lit environments such as a supermarket, the visibility of the laser light at the 670-680 nm wavelength suffers, particularly when far-out symbols located well away from the head are to be read.
It will be recalled that the laser beam is optically modified and focused to form a beam spot having a minimum beam cross-section or waist at a reference plane, and that a symbol can be read at either side of the reference plane. For ease of description, a symbol located between the reference plane and the head is defined as a "close-in" symbol, whereas a symbol that is located on the other side of the reference plane away from the head is defined as a "far-out" symbol. The term "close-in" symbol is also intended to cover the situation where the symbol actually is contacted by the head, or where the reference plane is located immediately outside the head. The range between minimum and maximum distances at which the system can read a symbol is often defined as the "depth of field". The depth of field is, of course, different for symbols of different densities.
With these definitions in mind, it will be appreciated that even marginally visible laser light (of wavelength 670-680 nm) can be readily seen on a close-in symbol, primarily because the speed of the beam spot across the symbol is slower for a close-in symbol than for a far-out symbol. However, such marginally visible laser light cannot, or at least not very readily, be positioned on a far-out symbol where the beam spot speed is faster.
It would be very desirable to have a laser diode-based system wherein both close-in symbols (including symbols contacting the head) and far-out symbols could be readily aimed at and read without the use of hunting techniques or aiming light arrangements.